I don't get it why people call him a problem for the case that you marry Mjoll. As far as I can tell from observing the two, Mjoll doesn't have any romantic feelings towards him, but considers him as a friend. And he is too shy to tell her his own feelings, which won't change after she is married to someone else. Sure, he'll suffer and be depressed, but he's too shy to actually tell his love/your wife about that.
So, what's the problem? Your wife dares to have a social life outside of your bedroom or what?
Most guys refuse to believe that once you're in the "Friend Zone" you're always in the "Friend Zone".

that's great she has a friend, but I don't want him living in my house
Well, if the only thing preventing your wife from cheating is having another guy live in the house then even if there is no other guy living in the house it's just a matter of time before she cheats elsewhere.

All that being said: it is a bit annoying and a little creepy that he's always around and I would probably arrange an accident had I married Mjoll, moved her and he moved
with us.
